A process is provided for determining the suitability of pyrolysis tar, such as steam cracker tar, for upgrading using hydroprocessing without long term fouling of the hydroprocessing reactor. The process includes heating a sample of the tar, quenching the sample, and measuring the total free radical content of the quenched sample. A pyrolysis tar can be blended with one having a lesser total free radical content to produce a blend that can be hydroprocessed with decreased fouling.